The client download is for the whole game client. Invited friends are not restricted on where to play. They may choose the play in the Lost Shores event or elsewhere in the game at their discretion. One of the reasons invites were made available today was so friends would have time to download the client before the event this weekend.

Eladrial is correct. Your friends will be able to start playing on November 15 (this Thursday) at 0900 Pacific time (that’s 1800 CET for those of you in Europe) and will be able to play until the event ends on Sunday evening (I believe this spills into the 19th for Europe). If they wish to continue playing after this time, they can buy a full copy of Guild Wars 2 and continue the progress they made during the weekend by applying the new game key to the existing account.